Today, I am working on my 1965 ARRL 6146B transmitter again. Right now, I only want to understand the correct operation of the CW function of the transmitter. I am performing these adjustments with the transmitter output connected to a power meter and 50 Ohm Heahtkit Cantenna dummy load.

“With a key plugged into J2, set S2 on OP. Set C4 (LOAD capacitor) at three quarters meshed, and switch S4 (meter switch) to read PLATE current. Watching the meter, close the key and swing C3 (plate capacitor) for a plate-current dip.”

[OK, no problem, the meter dips down to: 2.7 ma. which is really 135 ma. of actual Plate current]

“If the plate current dips below 170 ma. (indicated 3.4 ma.), decrease the capacitance in C4 (LOAD capacitor) and again tune C3 (PLATE capacitance) for resonance.The objective is to set the loading capacitor so that the plate current dips down to 170 ma.Check the grid current after the plate circuit is tuned; if it isn’t 2 ½ ma. Correct it by returning C1 (PLATE capacitor).”

(END OF ARRL HANDBOOK QUOTES)

The best I can achieve when maintaining the DRIVE at 2½ ma. and adjusting PLATE and LOAD capacitance is a plate current dip of 2.8 ma. on the meter, or 140 ma of actual Plate current.At this setting, the transmitter doesn’t always respond well to the keying. Sometimes I get some whoop and other times, the oscillator won’t start up which momentarily causes a large plate current draw!

BUT .. If I lower the DRIVE current to 1 ma., I can achieve 3.4 ma. of Plate current dip on the meter (170 mA of actual Plate current) right on the nose.

The transmitter behaves much better with the DRIVE current set to 1 ma.; the oscillator always starts right up, and the power out looks just as good as when the DRIVE is set to 2 ½ ma. (80 Watts)

My questions:

Why do you suppose the transmitter is behaving in this fashion?

Is it OK to run the 6146B when the transmitter is setup in this manner? (I would think that less DRIVE current is better for 6146B tube life?)

Insufficient grid current means insufficient negative bias, so you get more anode current. Basically, the valve is being pushed less hard into Class C. You need to investigate why the loading is not working properly.

I think your problem is not in the final stage, but is in the oscillator stage.

Obviously when running "straight through" they can interact a lot. If PA stage loading kills the oscillator, then plate current takes off into the stratosphere. Some similar designs (e.g. Eico 720) use a clamp tube.

If you double or triple in the final, you will need more drive but you will find less interaction between oscillator and PA.

It sounds to me that the oscillator circuit is being pulled down by the high DRIVE level. Once the drive level is lowered the oscillator becomes stable and is actually putting out more power to the PA. If the rig works at these levels, I don't see any problem. Just check for chirp which would indicate the oscillator being pulled.

I'd go back and investigate the metering circuit to see if the grid current reading is really what the meter says it is. Personally, I think there's a grid current metering error.

I'd go back and investigate the metering circuit to see if the grid current reading is really what the meter says it is. Personally, I think there's a grid current metering error.

That was my first take on this as well.

Use your DMM to see what's really going on, do not trust the built in metering unless and until you've proofed it.

Back in those days, when I was young and dad paid the bills, built a few homebrew xmtrs from circuits like that one in magazines, often leaving out the metering for the reasons of affordability. Filament bulb connected to the antenna terminals, tune for bright light, or, as we used to joke, "maximum smoke"... Actually, however wrong such might be, and I fully expect the same amount of forumboard warnings and posts for mentioning use of lightbulb as load, for the purposes of learning and experimenting, the light bulb actually taught me more than having the meter(s) on the front panel could have done. And I still got those things on the air and made plenty of contacts and even some exciting casual DX as well.

Would also add that a real transmitting tube like the 6146 was often unobtanium to the teenaged boy I was then. Had to alter many a design to utilize some Audio Output Tube or in later years even TV Sweep Tube or the likes and again, the learning experience proved priceless.

This kid subject many a three-five-eight TV crystal to such abuse, for every color set on somebody's front yard on pickup day coughed one out.

That was called, "QSY"...

Stripping those old TV sets, filling the carefully labeled cardboard junkboxes on the old wooden shelves in that inner city (I think we called 'em "ghettos" back then) basement, along with tubes, resistors, caps, xformers, spkrs, pilot lamps, sockets, even AC linecords, egg cartons instead of the little plastic drawers for sorting small components, I was happy as a pig in you-know-what.

Trimming crystals, well that takes me back. My Dad and I used to sell and install Marine Radio's on small boats on the South Coast UK. Crystals were never cheapif bought new, so we would watch out for surplus one's that were close at the emporium, and a bit of plate glass and Vim (cleaning powder) and gently grind them up. Oh Happy days.

Always used a neon to tune up a MW Marine TX, anyway was happy to find a neon still tunes up the 40 ext zepp just as easily.

Logged

ex MN Radio Officer, Portishead Radio GKA, BT Radio Amateur Morse Tester. Licensed as G3YCP ZL1DAB, now taken over my father (sk) call as ZL1BBW.

if the metering checks out but the issues remain then try the following:

that 150 pF going from the grid tank to the junction of the 1000 pF bypass and the6146 cathode pin 1...disconnect it from that junction and ground it. If no change thentry 500-1500 pF for that 150 pF and ground it.The way it is now, it returns the grid bypass to the cathode instead of to the ground...itlooks funky to me...

that 150 pF going from the grid tank to the junction of the 1000 pF bypass and the6146 cathode pin 1...disconnect it from that junction and ground it. If no change thentry 500-1500 pF for that 150 pF and ground it.The way it is now, it returns the grid bypass to the cathode instead of to the ground...itlooks funky to me...

Don't do this!

The 150 pf capacitor isn't a "bypass". It's part of the bridge neutralizing circuit. Do not increase its value!

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